Re: [PATCH bpf-next v1 02/19] bpf: implement an interface to register bpf_iter targets

[Yonghong Song <yhs@fb.com>]

On 4/28/20 9:20 AM, Martin KaFai Lau wrote:
> On Mon, Apr 27, 2020 at 01:12:36PM -0700, Yonghong Song wrote:
>> The target can call bpf_iter_reg_target() to register itself.
>> The needed information:
>>    target:           target name, reprsented as a directory hierarchy
>>    target_func_name: the kernel func name used by verifier to
>>                      verify bpf programs
>>    seq_ops:          the seq_file operations for the target
>>    seq_priv_size:    the private_data size needed by the seq_file
>>                      operations
>>    target_feature:   certain feature requested by the target for
>>                      bpf_iter to prepare for seq_file operations.
>>
>> A little bit more explanations on the target name and target_feature.
>> For example, the target name can be "bpf_map", "task", "task/file",
>> which represents iterating all bpf_map's, all tasks, or all files
>> of all tasks.
>>
>> The target feature is mostly for reusing existing seq_file operations.
>> For example, /proc/net/{tcp6, ipv6_route, netlink, ...} seq_file private
>> data contains a reference to net namespace. When bpf_iter tries to
>> reuse the same seq_ops, its seq_file private data need the net namespace
>> setup properly too. In this case, the bpf_iter infrastructure can help
>> set up properly before doing seq_file operations.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/bpf.h   | 11 ++++++++++
>>   kernel/bpf/Makefile   |  2 +-
>>   kernel/bpf/bpf_iter.c | 50 +++++++++++++++++++++++++++++++++++++++++++
>>   3 files changed, 62 insertions(+), 1 deletion(-)
>>   create mode 100644 kernel/bpf/bpf_iter.c
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 10960cfabea4..5e56abc1e2f1 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -31,6 +31,7 @@ struct seq_file;
>>   struct btf;
>>   struct btf_type;
>>   struct exception_table_entry;
>> +struct seq_operations;
>>   
>>   extern struct idr btf_idr;
>>   extern spinlock_t btf_idr_lock;
>> @@ -1109,6 +1110,16 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd);
>>   int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>>   int bpf_obj_get_user(const char __user *pathname, int flags);
>>   
>> +struct bpf_iter_reg {
>> +	const char *target;
>> +	const char *target_func_name;
>> +	const struct seq_operations *seq_ops;
>> +	u32 seq_priv_size;
>> +	u32 target_feature;
>> +};
>> +
>> +int bpf_iter_reg_target(struct bpf_iter_reg *reg_info);
>> +
>>   int bpf_percpu_hash_copy(struct bpf_map *map, void *key, void *value);
>>   int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value);
>>   int bpf_percpu_hash_update(struct bpf_map *map, void *key, void *value,
>> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
>> index f2d7be596966..6a8b0febd3f6 100644
>> --- a/kernel/bpf/Makefile
>> +++ b/kernel/bpf/Makefile
>> @@ -2,7 +2,7 @@
>>   obj-y := core.o
>>   CFLAGS_core.o += $(call cc-disable-warning, override-init)
>>   
>> -obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o
>> +obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o
>>   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
>>   obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o
>>   obj-$(CONFIG_BPF_SYSCALL) += disasm.o
>> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
>> new file mode 100644
>> index 000000000000..1115b978607a
>> --- /dev/null
>> +++ b/kernel/bpf/bpf_iter.c
>> @@ -0,0 +1,50 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/* Copyright (c) 2020 Facebook */
>> +
>> +#include <linux/fs.h>
>> +#include <linux/filter.h>
>> +#include <linux/bpf.h>
>> +
>> +struct bpf_iter_target_info {
>> +	struct list_head list;
>> +	const char *target;
>> +	const char *target_func_name;
>> +	const struct seq_operations *seq_ops;
>> +	u32 seq_priv_size;
>> +	u32 target_feature;
>> +};
>> +
>> +static struct list_head targets;
>> +static struct mutex targets_mutex;
>> +static bool bpf_iter_inited = false;
> The "!bpf_iter_inited" test below is racy.

Yes, as mentioned in the comments, all currently implemented
targets are called at __init stage (do_basic_setup()->do_initcalls()),
I think there is no race here. But looking at the
code again, I am not so sure about my assumption any more.

> 
> LIST_HEAD_INIT and DEFINE_MUTEX can be used instead.

Will use these macros instead. Thanks!

> 
>> +
>> +int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
>> +{
>> +	struct bpf_iter_target_info *tinfo;
>> +
>> +	/* The earliest bpf_iter_reg_target() is called at init time
>> +	 * where the bpf_iter registration is serialized.
>> +	 */
>> +	if (!bpf_iter_inited) {
>> +		INIT_LIST_HEAD(&targets);
>> +		mutex_init(&targets_mutex);
>> +		bpf_iter_inited = true;
>> +	}